The present invention relates to a focus control device, an endoscope system, a focus control method, and the like.
An endoscope system having a zoom function has been put to practical use, and configured so that the entire digestive tract (object) is observed on the wide-angle (WIDE) side (normal observation), and part of the digestive tract is observed in a zoom state on the telescopic (TELE) side (zoom observation) by adjusting the angle of view and the optical magnification of the objective lens using the zoom lens.
The endoscope system having a zoom function is normally configured to implement the optical magnification necessary for zoom observation through the lens design so that the angle of view of the objective lens becomes narrow (i.e., the optical magnification increases) and the best object distance decreases on the TELE side. The term “best object distance” used herein refers to the distance from the end of the objective lens to the object when the image position of the object coincides with the image plane of the image sensor. Since the object can be more closely observed by decreasing the best object distance on the TELE side, the optical magnification during zoom observation can be increased.
On the other hand, since the moving amount of the image position due to the movement of the object position increases as the best object distance decreases, the depth of field normally becomes shallow. In recent years, the depth of field of an endoscope system having a zoom function may be 1 mm or less on the TELE side, and it may be difficult for the user to bring the object into focus. JP-A-2002-258164, JP-A-8-106060 and JP-A-2002-253488 propose an endoscope system having an autofocus (AF) function in order to solve the above problem, for example. In JP-A-2002-258164, the AF operation is performed during normal observation and zoom observation, for example.